Method of producing samples for x-ray fluorescence analysis



United States Patent 3,546,452 METHOD OF PRODUCING SAMPLES FOR X-RAYFLUORESCENCE ANALYSIS Popko Reinder Dijksterhuis and Hermanus ArnoldusVerharen, Emmasingel, Eindhoven, Netherlands, assignors, by mesneassignments, to U.S. Philips Corporation, New York, N.Y., a corporationof Delaware No Drawing. Filed July 10, 1967, Ser. No. 652,027 Claimspriority, application Netherlands, July 9, 1966, 6609665 Int. Cl. G01n23/22 US. Cl. 250--51.5 4 Claims ABSTRACT OF THE DISCLOSURE A method ofproducing a sample suitable for X-ray analysis in which a mixture of thesubstance and a vitreous material is formed, melted, and after meltingis cooled to form a body. During cooling an electric current is passedthrough the body in the direction of a holder to free the body from theholder.

The invention relates to a method of producing samples for analysis,particularly samples to be examined by means of the X-ray fluorescencespectrometric analysing method (termed hereinafter the X-rayfluorescence analysis).

The examination-frequently a quantitative analysis of substances byX-ray fluorescence analysis requires the samples to be provided in anappropriate shape. There is known, for example, a method in which thesubstance to be examined, for example, a mineral, is mixed with borax,the mixture being heated until a homogeneous melt is obtained, which issubsequently cooled so that a vitreous body, the known borax bead, isobtained. For the X-ray fluorescence analysis this borax head shouldhave definite dimensions suitable for the geometry of the apparatusemployed for the analysis.

The borax bead is usually made in a crucible of graphite, platinum or aplatinum alloy.

A drawback in the production of analysis samples in this way is that thevitreous body (the borax bead) sticks to the Wall of the crucible.Efforts have been made to obviate this drawback by rapid cooling of thecrucible. The result thereof is adversely affected, when the bottom and/or the wall of the crucible are rough. A further disadvantage resides inthat owing to the rapid cooling strong local stresses are produced inthe vitreous body, which may give rise to cracking.

It has furthermore been attempted to diminish the adhesion of the beadto the crucible by making a crucible of a given material, such asplatinum-gold and platinumgold-rhodium alloys. However, this could notcompletely obviate the drawback.

There has now been found a method in which said adhesion of the vitreousbody to the crucible does not occur or is obviated, a bead beingobtained, in which local stresses are reduced.

This is highly important for permitting automatic X- ray fluorescenceanalysis, which would otherwise not be feasible.

It has been found that the adhesion of the vitreous body to the cruciblecan be obviated by passing electric current through the areas ofadhesion from the body towards the crucible.

The invention relates to a method of producing samples for analysis, inwhich a mixture of the substance to be tested and a substance, fromwhich a vitreous body is formed by melting and subsequent cooling, isheated in or on a holder of electrically conductive material until themixture is melted, after which it is cooled, and is characterized inthat during the cooling of the vitreous body electric current is passedthrough the body towards the holder until the vitreous body no longeradheres to the holder.

The current may be passed through the vitreous body towards the holderin various ways. The holder may be connected to the negative terminal ofa direct-voltage source, while an electrode, for example, of graphitewool, connected to the positive terminal, is pressed against thevitreous body. It has been found that the desired effect may furthermorebe obtained by means of alternating current. In the interface betweenthe vitreous body and the holder a blocking effect is produced so that,when alternating voltage is used, current is passed only in onedirection, i.e. from the body to the holder.

The voltage to be used for the passage of current depends to some extentupon the material used for the vitreous body. It depends more intimatelyupon the temperature of said body at which the current starts passingthrough, owing to the intimate relationship between its resistance andtemperature. It is advantageous to start the passage of current at atemperature at which the vitreous body is still viscous; with thefrequently employed borax bead this is at about 600 C. to 700 C. In thiscase a voltage of 110 to 220 v. is suitable.

It has been found that the current can be advantageously passed throughthe vitreous body to the holder by means of spark discharge or voltageflash-over.

The spark discharge or the voltage flash-over may be achieved in knownmanner, for example, by holding a pointed electrode, connected to avoltage source of, for example, 50 kv., at a given distance, forexample, 2 ems. above the vitreous body and by connecting the crucibleto earth. The voltage flash-over has to take place through the beadtowards the crucible. If the spark discharge is produced by directvoltage, the electrode above the vitreous body has to be connected tothe positive terminal of the voltage source. It has been found that thespark discharge may also be produced by means. of alternating voltage.Due to the blocking effect in the interface between the vitreous bodyand the crucible the flash-over is also in this case produced only inone direction. The process of the disengagement of the vitreous bodyduring the spark discharge can be observed visually and it can beassessed when the process has finished. It can thus be ascertained in asimple manner when the spark discharge can be finished.

It is advantageous to start the spark discharge, when the resistance ofthe vitreous body is still fairly low; this is the case, when thevitreous body is still at a higher temperature. Therefore, it ispreferred to start the spark discharge when the vitreous body is stillviscous. If borax is used, this is at 600 C. to 700 C. The discharge maybe terminated, for example, at about 200 C. The spark discharge may alsobe started at a lower temperature than about 700 C. These values are notvery critical. Also for this reason the method according to theinvention contributes to the possibility of carrying out the X-rayfluorescence analysis automatically. Automation of the X-rayfluorescence analysis is essential for many industries, inter alia forthe cement industry, in connection with the fact that frequently a verygreat number of samples has to be examined within a short period oftime.

The holder is usually a crucible of conventional material, such asplatinum, platinum-gold and graphite (electro-graphite, pyrolyticgraphite). The substance forming, subsequent to melting and cooling, avitreous body may be a known substance, for example, analkali-tetra-borate, for instance borax, a phosphate and a bisulphate.

The method according to the invention will be explained with referenceto the following examples.

(1) 4 parts by weight of borax were mixed with 1 part by weight of limeand stone (the substance to be tested). The mixture was melted in acrucible of platinum-gold (-5) by heating at about 1000 C. After themass had completely melted, it was cooled in air. Above the melt wasarranged a pointed electrode, connected to an alternating-voltage sourceof 50 kv., so that the distance between the electrode and the surface ofthe melt was about 2 ems. The crucible was connected to earth. The sparkdischarge was started at a temperature of about 700 C. of the vitreousbody formed from the melt; during cooling the discharge was continueduntil the vitreous body had completely disengaged from the wall of thecrucible. This took about 3 minutes.

(2) As described under (1), 4 parts by weight of sodium phosphate and 1part by weight of cement were used to make a sample for analysis in agraphite crucible. The spark discharge was obtained by means of adirect-voltage source. The crucible was connected to the negativeterminal.

(3) As described under (1), 4 parts by weight of borax and 1 part byweight of lime sand stone were used for preparing a sample for analysisin a platinum crucible. An electrode of graphite wool, connected to thepositive terminal of a direct-voltage source of 220 v. was pressedagainst the surface of the sample which had become viscous when cooledto about 650 C. The crucible was connected to the negative terminal. Thecurrent was passed through during the cooling of the sample until it haddisengaged from the crucible. This occurred when the temperature of thesample (the vitreous body) was about 250 C.

The method can be improved in that during the spark discharge the latteris surrounded by a stream of a gas, for example air. The direction ofthe stream should be from the electrode to the holder of the sample.This intprovement effects that the glass bead is disengaged from theholder in a shorter period of time. In many examples the disengagementof the sample was effected within 30 seconds.

What is claimed is:

1. A method of producing samples for X-ray fluorescence analysiscomprising the steps of forming a mixture of a substance to be testedand a substance from which a vitreous body is formed by melting, heatingsaid mixture while in contact with a holder of electrically conductivematerial until the mixture is melted, thereafter cooling the meltedmaterial to form a vitreous body, and passing an electric currentthrough this body during cooling toward the holder until the vitreousbody no longer adheres to the holder.

2. A method as claimed in claim 1, in which the current starts passingthrough before the vitreous body has completely solidified.

3. A method as claimed in claim 2, in which the electric current ispassed through the body by means of spark discharge.

4. A method as claimed in claim 3, in which the spark discharge issurrounded by a streaming gas.

References Cited UNITED STATES PATENTS WILLIAM F. LINDQUIST, PrimaryExaminer

